This invention relates to antennas and in particular to a center fed reflector radar antenna having very low sidelobes.
One of the most widely used microwave antennas for radar is a parabolic reflector, which is a device that radiates and focuses electromagnetic energy by use of the shape of the curve of a parabola. The typical design of a radar system with a parabolic reflector involves an individual radiator that transmits energy toward the reflector where it is then directed toward a target. Reflected energy from the target returns to the parabolic reflector where it is coupled to a receiver for processing. The lobe structure of the antenna radiation pattern outside the major lobe (main beam) region usually consists of a large number of minor lobes, of which those adjacent to the main beam are sidelobes. Sidelobes can be a source of problems for a radar system. In the transmit mode they represent wasted radiated power illuminating directions other than the desired main beam direction, and in the receive mode they permit energy from undesired directions to enter the system. The text "Radar Handbook", second edition, Merrill Skolnik, McGraw-Hill Inc., 1990, provides an overview of the art of reflector antennas in Chapter 6. It is well known in the antenna art that a center fed reflector antenna is limited in terms of the minimization of sidelobe levels due to forward scattered energy from the feed and its support structure.
In the prior art, the use of struts or spars with ogival cross-sections to provide a substantial reduction in "backscatter" in an antenna is shown and described in U.S. Pat. No. 3,419,371, issued Dec. 31, 1968, to Albert Cohen et al. and assigned to Communication Structures, Inc. The descriptive term ogival is used to describe a geometric figure formed by an arc drawn symmetrically on appropriate sides of its chord which looks like an oval with pointed ends. The spars are positioned so that one pointed end or edge faces the radar reflector and the opposite end or edge faces away from the reflector. Electromagnetic waves upon striking one of the sharp edges apparently flow around the surface of the spar as traveling waves and meet again at the opposite edge. The amount of backscattering disruption that occurs depends on the shape of the ogive and the path length of the traveling wave component. Using a method of moment analysis, the backscattering can be minimized by varying the ogive shape.
The use of a dual mode conical horn to suppress sidelobes is described in an article entitled "A New Antenna With Suppressed Sidelobes" by P. D. Potter, Microwave Journal, June 1963, pp. 195-202. The dual mode conical horn utilizes a conical horn excited at the throat region with a step discontinuity in both the dominant TE.sub.11 mode and the higher-order TM.sub.11 mode. These two modes are then excited in the horn aperture with the appropriate relative amplitude and phase to effect sidelobe suppression and beamwidth equalization. A stepless dual mode horn which also converts TE.sub.11 to TM.sub.11 energy in a horn is described in the text "Antenna Engineering Handbook", (second edition), Richard C. Johnson and Henry Jasik, Editors, McGraw-Hill, Inc., 1984, Chapter 15.
Typically the realization of low sidelobes in reflector antennas is achieved through the use of offset reflector configurations. The intent is to remove the scattering blockage of the feed and struts from in front of the radiating aperture of the reflector. Unfortunately, the manufacturing costs of the offset configurations are greater than for the center fed reflector approach. Prior art experience with center fed reflector antennas has indicated sidelobe levels of -25 dB or greater with respect to the main lobe amplitude.